Interferometry for the characterization and diagnostics of targets and plasma in laser-plasma experiments
Recent progresses on laser wakefield acceleration (LWFA) open the way for the development of novel compact accelerators with potential applications for example in biomedical therapy and diagnosis, as well as novel high-energy particle accelerators. For a practical application of such sources the full control of the LWFA process is necessary. Among the fundamental parameters in the acceleration process there are the particle density in the gas targets (e.g., cells, jets and capillaries) and the electron density of the plasma thereby created by the laser, both of which can be accurately measured by interferometric techniques.
Two interferometric methods are implemented for the characterization of: 1) a pulsed gas jet; 2) a flowing gas cell.
1) A Nomarski interferometer is adopted to study the spatial particle density distribution in a freely expanding pulsed gas jet in vacuum. The valve has a rectangular orifice 1.2 mm wide of 10 mm length, with a de Laval cross section profile. The valve is placed in a vacuum chamber with 10 mbar background pressure, and argon, nitrogen and carbon dioxide gases are used. The measurements are performed along the long side of the jet at different backing pressure, and the evolution of the particle density in the gas jet during the few-ms long pulse is investigated. As example Figure 1 shows an interferogram recorded using an Argon gas jet with 30 bar backing pressure after 1 ms from the valve opening. Nomarski interferometry therefore provides a useful method to measure the particle density distribution in a pulsed gas jet used for LWFA experiments.
2) A single-arm two-colour interferometer, so-called second harmonic interferometer (SHI), is used to study the particle density dynamics inside a gas flow cell placed in vacuum. The flow gas cell is provided by the company SourceLAB (Paris-FRANCE) that develops targets specifically for LWFA systems. A pilot characterization of the argon gas particle density evolution pulsing the gas in the cell is performed to demonstrate the feasibility of second harmonic interferometry to monitor on-line the particle density within the cell. Measurements have been performed ad various backing pressure (0.2 bar, 0.4 bar, 0.6 bar and 0.8 bar) and using three set of nozzles with different apertures (0.2 mm, 0.5 mm and 2 mm). As example, Figure 2 shows the particle density time evolution from the opening of the electro valve providing the gas to the cell measured with 0.8 bar backing pressure for the three nozzle diameters. The SHI satisfies the requirements for a fast and reliable measurement, being intrinsically stable and combining a high-sensitivity with a high-speed.